1. Field of the Invention
The present invention relates to the irradiation of materials in liquid or gaseous form, and in particular to the irradiation of wastes or drinking water with high-energy electrons.
2. Description of the Prior Art
High-energy electron beams are being used in many industrial processes and for sterilization of materials such as surgical equipment and some food products. A new area of application of electron beams is the treatment of gaseous or liquid wastes, not only for sterilization, but also to break up toxic or otherwise undesirable components. Electron irradiation techniques can also be applied to sterilization of drinking water. In the case of gaseous wastes the gas may pass through a "curtain" of electrons formed by scanning the pencil-thin beam emerging from an accelerator rapidly with an oscillating magnetic or electric field. Alternatively, the beam may be sufficiently spread out by scattering in the gas to cover the cross section of a smoke stack.
The order-of-magnitude of the electron energy required for the aforementioned applications of electron irradiation is determined by the nature of the material irradiated. Since the range of 500 keV electrons in air is approximately 1.5 meters, irradiation of the aforementioned gaseous materials may be accomplished with electrons having an energy on the order of 500 keV. Since the range of 2-MeV electrons in water is about 9.5 millimeters, water may be treated with electrons having an energy on the order of 2-MeV if a relatively shallow river or thin waterfall is formed. For this system to be practical it is necessary
(a) that the electron beam be very intense (hundreds of milliamperes), and PA1 (b) that it can be scanned rapidly across a relatively wide area.
The beam can be scanned in vacuum as it is emerging from the accelerator and thereafter can be made to pass through a thin metallic window, or it can be directed through a small orifice into air or partial vacuum to be scanned immediately thereafter. Multiple scattering by air molecules causes the beam to increase in diameter quite rapidly in air at atmospheric pressure, and so it is important that the scanning mechanism be relatively close to the orifice of emergence. Clearly, the vacuum system has to have several stages of differential pumping between the aforementioned orifice and the high vacuum accelerator section for the electrons. This technique is well known and utilized in electron-beam welding equipment.
For high-volume processes, such as the treatment of waste water or drinking water, a major limitation is that of the water flow, which must be uniform and fast. The need for high flow rates may be seen from the following analysis, which shows that the specific treatment costs ($/gallon) are significantly reduced if the facility treats larger quantities of liquid. Such a facility requires high power electron beam machines, which typically have greatly reduced specific capital equipment costs expressed in $/watt.
The cost to treat 1 liter of liquid is comprised of three components: the energy cost, the capital cost and the maintenance cost. The energy cost is determined by the required dose to achieve a certain kill ratio. For a typical dose of 100 krad (1 kilogray), it is necessary to treat each gram of water with 1 Joule of E-beam energy. Therefore, to treat 1 liter of water an energy of 1 kJ or 2.78 E-4 kWh is required. If one further considers the efficiency of energy input to energy delivered to the liquid, one arrives at an energy requirement in the vicinity of 5 E-4 kWh. If one then assumes an energy cost of 8 cents per kWh, one obtains an energy cost per liter of 4 E-5 $/liter.
The capital costs vary greatly with the size of the installation. For example, a 10 kW installation might have specific costs of 50$/W, while a 1 MW installation would have specific costs of only $5/W. A 10 kW installation is capable of treating 20,000 l/hr or 138 million liters per year, assuming a utilization factor of 80%. For an equipment cost of $500,000, and assuming 12% for interest and depreciation one arrives at a yearly capital cost of $60,000 or at a specific cost of 4.35 E-4 $/l. The specific capital costs for a 1 MW installation are, on the other hand, only 4.35 E-5 $/l. This show that for small installations, the energy and capital costs are approximately equal, while for large installations the energy costs dominate.
The specific maintenance costs are again much higher for a small installation, as both installations require the need of one well trained technical person.
In addition to the foregoing economic considerations, considerations of energy loss by an electron beam passing through the atmosphere also favor high beam currents. An electron beam emerging into the atmosphere loses energy by collisions with air molecules but, in turn, also heats the gas it encounters. A high current beam will heat the gas more than a low current beam, thus it will more strongly reduce the air density and suffer, percentage-wise, less energy loss than a low current beam. Furthermore, the high current beam will spread less as it travels in lower density gas. An analysis of this phenomenon in some detail is presented in a paper by S. Philp entitled "Heating of the Air by the E-beam and its Effect on Energy Loss and Scattering", a copy of which has been designated "Exhibit A" and is filed with this application.
For processes other than the foregoing high-volume processes, such as the treatment of "hazardous waste" such as chemical waste, the required exposure is higher and therefore the throughput will be less. One purpose in this connection is to break up solvents in the hazardous waste, such as carbon tetrachloride. A dose of about a megarad is required, but there is no flow limitation.